Belt drives comprising tension rollers for providing tension to the belt have been known for a long time and the application of such belt drives in combine harvesters has also been known for a long time. These belt drives are used to drive the threshing drum of a combine harvester. The threshing drum beats the cut crops to break the crop, and subsequently shakes the grains away from their stalks.
GB1480566, U.S. Pat. No. 3,550,463 and U.S. Pat. No. 4,324,552 disclose earlier concepts of such belt drives including tension rollers.
Apart from the tension roller assemblies disclosed in GB1480566, U.S. Pat. No. 3,550,463 and U.S. Pat. No. 4,324,552, the present applicant currently manufactures a further version of a tension roller assembly. The tension roller assembly is in particular used for a combine harvester having twin rotors, and a dynamic feed roll. The dynamic feed roll is a drum which removes stones and accelerates the crop into the twin rotors. This tension roller assembly is further elucidated in the figures and the description of the present document.
The belt drive of the currently manufactured system has two gear ratios, one for low speed operation and one for high speed operation of the rotating drum. This is required because a combine harvester should be able to harvest different crops having different characteristics. To this end, the rotating drum should be able to rotate at two different speeds.
The currently manufactured system has two gear ratios which are formed by two different driven pulleys of the belt drive, i.e. a driven pulley with a small diameter for the high gear ratio and a driven pulley with a large diameter for the low gear ratio. This allows the belt drive to be operated at two different speeds having two different maximal torques. Because the driven pulleys have different diameters, each gear ratio requires its own belt of a required length. When switching gear ratios, the user also has to exchange one belt for the other belt.
In order to switch gear ratios, it is a requirement to be able to remove the tension from the belt by disengaging the tension roller from the belt. This is also necessary in case of replacement of a damaged or worn-out belt. To this end, the tension roller is incorporated in a tension roller assembly which allows for a movement of the tension roller between an engaged position and a disengaged position from the belt. A spring, generally a compression spring, provides the tension of the roller onto the belt.
The currently manufactured system is equipped with a tension roller assembly having an overcenter assembly which allows the tension roller to be moved between the engaged position and the disengaged position. The current tension roller assembly comprises a support arm which supports the tension roller. The tension roller assembly further comprises a tension arm which is connected to the support arm at one end. The tension arm is connected to a frame at the other end via an overcenter assembly. The overcenter assembly is pivotably mounted to the frame.
When the tension roller is moved from the engaged position to the disengaged position, it has to move beyond the center position of the overcenter assembly. During the movement from the engaged position to the center position, the spring is further compressed, and the movement is therefore carried out against the compression force of the spring. When the overcenter assembly passes the center position, the spring starts to extend and during a further movement from the center position to the disengaged position, the pulley is retracted from the belt, thereby relieving the belt from its tension.
The system which is currently manufactured has a number of drawbacks.
First, the system is relatively unpractical for a user who wishes to switch the tension roller assembly between the engaged position and the disengaged position. The rotation of the overcenter assembly between the engaged position and the disengaged position is carried out under the force of the spring, and the spring force may be rather high. This results in a strong force on the user. The rotation is generally carried out with a wrench. Because of the high spring force, the hand of the user may get stuck between the wrench and the machine. In extreme cases, this may even lead to an injury of the user.
Further, the present assembly has relatively little room for the belt to be removed from the belt drive when the tension roller is in the disengaged position. This is a result of the specific configuration of the present assembly. The limited space makes it difficult for a user to exchange one belt for the other belt.
Further, during operation the belt generally stretches somewhat as a result of pulling forces. The stretching results in an extension of the spring which pushes the tension roller against the belt. The stretching may become particularly large when large, unevenly distributed packages enter the combine, causing high torque peaks on the belt drive.
The stretching in turn results further inward position of the tension roller, caused by an extension of the spring. The extension of the spring results in a lower spring force and a lower force exerted by the tension roller and thus, in a lower tension of the belt. This lowers the torque which can be applied by the belt drive and results in earlier slipping of the belt and, eventually, blockage of the dynamic feed roller.